Generally, the driving circuit of speakers employs class-D amplifier or other power MOS driving scheme, most of which is implemented by an H-bridge configuration, for example, see U.S. Pat. Nos. 6,211,728 and 7,209,003. For headphones, due to the consideration for acoustic fidelity requirements, the driving circuit needs both positive and negative voltage sources, and thus often employs a charge pump to generate the needed negative voltage, for example, see U.S. Pat. Nos. 7,061,328 and 7,701,294.
As shown in FIG. 1, for driving a speaker 10, the conventional H-bridge 12 includes switches SW1 and SW2 connected in series between voltage input terminals 14 and 16, and switches SW3 and SW4 connected in series between voltage input terminals 18 and 20. By switching the switches SW1, SW2, SW3 and SW4, a voltage Vout is generated between switch nodes 22 and 24 for driving the speaker 10. FIG. 2 shows a conventional charge pump, which includes switches SW5 and SW6 connected in series between voltage input terminals 26 and 28, switches SW7 and SW8 connected in series between a voltage output terminal 30 and a voltage input terminal 32, and a flying capacitor Cflying connected between switch nodes 34 and 36. By switching the switches SW5, SW6, SW7 and SW8, the positive supply voltage VDD is converted into a negative voltage −VDD at the voltage output terminal 30.
Many commercial sound outputting apparatuses are capable of driving a speaker and a headphone, which use an H-bridge for driving the speaker and a charge pump for driving the headphone, for example, see U.S. Pat. No. 7,515,980. Therefore, such device has eight switches and thus requires larger circuit area and higher costs.